NAME

DESCRIPTION

This page covers the very basics of understanding, creating and using regular expressions ('regexes') in Perl.

The Guide

Simple word matching

The simplest regex is simply a word, or more generally, a string of characters. A regex consisting of a word matches any string that contains that word:

"Hello World" =~ /World/; # matches

In this statement, World is a regex and the // enclosing /World/ tells perl to search a string for a match. The operator =~ associates the string with the regex match and produces a true value if the regex matched, or false if the regex did not match. In our case, World matches the second word in "Hello World", so the expression is true. This idea has several variations.

In the last regex, the forward slash '/' is also backslashed, because it is used to delimit the regex.

Non-printable ASCII characters are represented by escape sequences. Common examples are \t for a tab, \n for a newline, and \r for a carriage return. Arbitrary bytes are represented by octal escape sequences, e.g., \033, or hexadecimal escape sequences, e.g., \x1B:

With all of the regexes above, if the regex matched anywhere in the string, it was considered a match. To specify where it should match, we would use the anchor metacharacters ^ and $. The anchor ^ means match at the beginning of the string and the anchor $ means match at the end of the string, or before a newline at the end of the string. Some examples:

Using character classes

A character class allows a set of possible characters, rather than just a single character, to match at a particular point in a regex. Character classes are denoted by brackets [...], with the set of characters to be possibly matched inside. Here are some examples:

In the last statement, even though 'c' is the first character in the class, the earliest point at which the regex can match is 'a'.

/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
/yes/i; # also match 'yes' in a case-insensitive way

The last example shows a match with an 'i'modifier, which makes the match case-insensitive.

Character classes also have ordinary and special characters, but the sets of ordinary and special characters inside a character class are different than those outside a character class. The special characters for a character class are -]\^$ and are matched using an escape:

If '-' is the first or last character in a character class, it is treated as an ordinary character.

The special character ^ in the first position of a character class denotes a negated character class, which matches any character but those in the brackets. Both [...] and [^...] must match a character, or the match fails. Then

\w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]

\D is a negated \d; it represents any character but a digit [^0-9]

\S is a negated \s; it represents any non-whitespace character [^\s]

\W is a negated \w; it represents any non-word character [^\w]

The period '.' matches any character but "\n"

The \d\s\w\D\S\W abbreviations can be used both inside and outside of character classes. Here are some in use:

/\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
/[\d\s]/; # matches any digit or whitespace character
/\w\W\w/; # matches a word char, followed by a
# non-word char, followed by a word char
/..rt/; # matches any two chars, followed by 'rt'
/end\./; # matches 'end.'
/end[.]/; # same thing, matches 'end.'

The word anchor\b matches a boundary between a word character and a non-word character \w\W or \W\w:

In the last example, the end of the string is considered a word boundary.

Matching this or that

We can match match different character strings with the alternation metacharacter '|'. To match dog or cat, we form the regex dog|cat. As before, perl will try to match the regex at the earliest possible point in the string. At each character position, perl will first try to match the the first alternative, dog. If dog doesn't match, perl will then try the next alternative, cat. If cat doesn't match either, then the match fails and perl moves to the next position in the string. Some examples:

At a given character position, the first alternative that allows the regex match to succeed wil be the one that matches. Here, all the alternatives match at the first string position, so th first matches.

Grouping things and hierarchical matching

The grouping metacharacters () allow a part of a regex to be treated as a single unit. Parts of a regex are grouped by enclosing them in parentheses. The regex house(cat|keeper) means match house followed by either cat or keeper. Some more examples are

Extracting matches

The grouping metacharacters () also allow the extraction of the parts of a string that matched. For each grouping, the part that matched inside goes into the special variables $1, $2, etc. They can be used just as ordinary variables:

In list context, a match /regex/ with groupings will return the list of matched values ($1,$2,...). So we could rewrite it as

($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

If the groupings in a regex are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis, etc. For example, here is a complex regex and the matching variables indicated below it:

/(ab(cd|ef)((gi)|j))/;
1 2 34

Associated with the matching variables $1, $2, ... are the backreferences\1, \2, ... Backreferences are matching variables that can be used inside a regex:

/(\w\w\w)\s\1/; # find sequences like 'the the' in string

$1, $2, ... should only be used outside of a regex, and \1, \2, ... only inside a regex.

Matching repetitions

The quantifier metacharacters ?, *, +, and {} allow us to determine the number of repeats of a portion of a regex we consider to be a match. Quantifiers are put immediately after the character, character class, or grouping that we want to specify. They have the following meanings:

a? = match 'a' 1 or 0 times

a* = match 'a' 0 or more times, i.e., any number of times

a+ = match 'a' 1 or more times, i.e., at least once

a{n,m} = match at least n times, but not more than m times.

a{n,} = match at least n or more times

a{n} = match exactly n times

Here are some examples:

/[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
# any number of digits
/(\w+)\s+\1/; # match doubled words of arbitrary length
$year =~ /\d{2,4}/; # make sure year is at least 2 but not more
# than 4 digits
$year =~ /\d{4}|\d{2}/; # better match; throw out 3 digit dates

These quantifiers will try to match as much of the string as possible, while still allowing the regex to match. So we have

The first quantifier .* grabs as much of the string as possible while still having the regex match. The second quantifier .* has no string left to it, so it matches 0 times.

More matching

There are a few more things you might want to know about matching operators. In the code

$pattern = 'Seuss';
while (<>) {
print if /$pattern/;
}

perl has to re-evaluate $pattern each time through the loop. If $pattern won't be changing, use the //o modifier, to only perform variable substitutions once. If you don't want any substitutions at all, use the special delimiter m'':

$pattern = 'Seuss';
m'$pattern'; # matches '$pattern', not 'Seuss'

The global modifier //g allows the matching operator to match within a string as many times as possible. In scalar context, successive matches against a string will have //g jump from match to match, keeping track of position in the string as it goes along. You can get or set the position with the pos() function. For example,

Search and replace

Search and replace is performed using s/regex/replacement/modifiers. The replacement is a Perl double quoted string that replaces in the string whatever is matched with the regex. The operator =~ is also used here to associate a string with s///. If matching against $_, the $_ =~ can be dropped. If there is a match, s/// returns the number of substitutions made, otherwise it returns false. Here are a few examples:

With the s/// operator, the matched variables $1, $2, etc. are immediately available for use in the replacement expression. With the global modifier, s///g will search and replace all occurrences of the regex in the string:

The last example shows that s/// can use other delimiters, such as s!!! and s{}{}, and even s{}//. If single quotes are used s''', then the regex and replacement are treated as single quoted strings.

The split operator

split /regex/, string splits string into a list of substrings and returns that list. The regex determines the character sequence that string is split with respect to. For example, to split a string into words, use